Transformers, Switchgear, and Distribution Equipment
Transformer fundamentals
Know the turns ratio (V₁/V₂ = N₁/N₂), the difference between additive and subtractive polarity, and cooling class designations: ONAN, ONAF, OFAF. Nameplate kVA is based on a stated temperature rise and ambient; forcing more cooling raises usable capacity. Inrush current on energization can reach 8–12 times rated current for a few cycles and is rich in second harmonic - which is exactly what harmonic-restraint differential relaying uses to avoid tripping.
Paralleling conditions
Transformers may be paralleled only with the same voltage ratio, same polarity, same phase-angle displacement (vector group), and compatible impedances. Impedance mismatch causes load to divide inversely with percent impedance - the lower-impedance unit carries more.
Worked example
Two 1000 kVA units at 5.0% and 6.5% impedance share 2000 kVA of load. Load divides as 6.5:5.0, so the 5.0% unit carries 1.3× the other: roughly 1130 kVA versus 870 kVA. The lower-impedance transformer is overloaded at 113% while the total stays within 2000 kVA. That is why impedance matching within about 10% is a practical rule.
Switchgear and distribution
Understand breaker ratings - nominal voltage, continuous current, short-circuit interrupting rating, close-and-latch - and that interrupting rating must exceed calculated available fault current including motor contribution. Know metal-clad versus metal-enclosed construction, drawout versus fixed breakers, and the function of a Pad-mounted versus vault installation. Sectionalizing and tie breakers change the fault current seen downstream; always evaluate the worst-case configuration.